are payments processors mutually exclusive?

I'm bootstrapping a software business for the first time. I worked in 'stealth mode' for one full-year and now I've got about 100 private-beta-testers loving the software (and already begging for rebates once it goes 1.0 ;) and I'll soon start working on payment processing, which brings to my question(s)...

For credit cards, I'm considering FastSpring which I've heard good thing about (both here and elsewhere) and I haven't found much on the following subject: are payments processors like FastSpring mutually exclusive with regards to other payments processors or to ewallets like Neteller, MoneyBookers, PayPal, etc.?

Can I legally use, say, FastSpring to process all the credit cards payments and process PayPal, Neteller and MoneyBookers directly myself?

What about using, say, FastSpring, to process one credit card payment out of two and another credit card processor to processor the other? (It's a theoretical question, to better understand what is at work here).

Can payments processors companies legally force exclusive agreements? Are some doing it?

No, they're not exclusive (and if any suggest that they're giving you an 'exclusive' deal, run away).

Some people have a second processor as a backup in case the first one is down. If you do sign up with more than one, then there's nothing to stop you routing some transactions to one and some to the other.

Denis' answer is right on.

For your benefit, the more you'll process with any one processor, the better rates they'll give you. Thus, as you ramp up sales, go back and get new quotes from them (and others) every 12 months or so. Don't sign any contract locking you in. For this "volume discount" reason, it doesn't make too much sense to spread them around.

Having FastSpring and PayPal should be all you need (and perhaps Google Cart, but I believe their rates are higher than PayPal, in which case leave them out).

Best of luck!